res/drawable - Here we store the various density-independent graphic assets used in our application.

res/drawable-hdpi - Series of folders for density specific images to use for various resolutions.

res/mipmap - most commonly used for application icons. See this section for more details.

The most frequently edited files are:

res/layout/activity_foo.xml - This file describes the layout of the activity's UI. This means the placement of every view object on one app screen.

src/.../FooActivity.java - The Activity "controller" that constructs the activity using the view, and handles all event handling and view logic for one app screen.

AndroidManifest.xml - This is the Android application definition file. It contains information about the Android application such as minimum Android version, permission to access Android device capabilities such as internet access permission, ability to use phone permission, etc.